This invention relates to an effective technique for automating an arc welding apparatus.
With the progress of automation of welding apparatuses, various techniques have been proposed, as disclosed in Japanese Patent Laid-Open Publications Nos. HEI-7-148576 entitled xe2x80x9cNon-Consumable Electrode Automated Arc Welding Processxe2x80x9d and HEI-10-249526 entitled xe2x80x9cTIG Welding Apparatus for Rotating Bodyxe2x80x9d.
The automated arc welding process of Japanese Patent Laid-Open Publication No. HEI-7-148576 shows a distance measuring method during welding process in which a welding torch driven under constant arc length control or constant voltage control is provided with a laser sensor for measuring the distance from a base metal.
As is well known, an electric arc formed across the electrode and the base metal emits very bright light, which may cause an operation error of the laser sensor. To deal with this problem, in the aforementioned automated arc welding process sensing of the base metal by the laser sensor is performed at a position remote from the electrode. However, since the distance between a tip end of the electrode and a weld metal (molten pool) is important for welding, such a control, which is based on the distance between the electrode and the base metal, hardly achieves desired control accuracy. An attempt to bring the laser sensor close to the electrode for increasing the control accuracy will result in an operation error of the laser sensor because light from the electric arc (hereinafter referred to for brevity as xe2x80x9carc lightxe2x80x9d) projects in the sensing field of the laser sensor. Thus, the foregoing automated arc welding process still has a problem of insufficient welding control accuracy.
The welding apparatus disclosed in Japanese Patent Laid-open Publication No. HEI-10-249526 includes a slit light source and a television camera. The slit light source illuminates a bevel or groove, and an image of the groove is picked up by the television camera and in turn is subjected to image processing so that control in a direction of the width of the groove is performed according to a result of the image processing. The vertical axis of the welding apparatus is controlled in a direction of the depth of the groove on the basis of weld voltage. Due to the arc light and weld spatters having high illumination intensities, which are present in the field of the television camera together with the groove illuminated with the slit light source, a special technique is required to selectively take up an image of the groove alone. This requirement increases cost of the welding apparatus. Furthermore, height control of the electrode achieved in reliance on the control of the arc voltage gives rise to a problem, as discussed below.
The arc voltage control relies on a physical phenomenon in which the length of an electric arc (hereinafter referred to for brevity as xe2x80x9carc lengthxe2x80x9d) is nearly in proportion to the voltage between the electrode and the base metal (hereinafter referred to for brevity as xe2x80x9celectrode to base metal voltagexe2x80x9d). In the arc voltage control, the electrode to base metal voltage is measured to estimate an arc length based on a measured value, and vertical movement of the electrode is controlled so that the estimated arc length is in equal to a desired value.
However, an experiment made by the present inventors have proved that under a certain condition, the arc voltage control is difficult to achieve regardless of whether the arc is a pulsed arc or a continuous arc.
FIGS. 8A and 8B, respectively, show an experimental apparatus used for carrying out the conventional pulsed arc voltage control, and a graph showing the experimental result obtained using the experimental apparatus.
The experimental apparatus shown in FIG. 8A was arranged such that a welding source 101 produces an electric arc between a base metal 102 and an electrode 103 and, at the same time, a recorder 105 via a filter 104 records the arc voltage. Welding conditions utilized a reference current set to 4 A, a peak current set to 20 A, a pulse frequency set to 2 Hz, and a duty cycle set to 40%.
For purposes of illustration, the electrode 103 is shown as taking three different operation modes designated by reference characters xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d, respectively. The number of the electrode 103 used was only one. From xe2x80x9cAxe2x80x9d point to xe2x80x9cBxe2x80x9d point, welding was carried out by moving a torch (not shown) rightward in FIG. 8A over a distance of about 20 mm while moving the torch upward at a constant speed such that the arc length LA at xe2x80x9cAxe2x80x9d point is 0.3 mm and the arc length at xe2x80x9cBxe2x80x9d point is 1.0 mm. From xe2x80x9cBxe2x80x9d point to xe2x80x9cCxe2x80x9d point, the welding further continued by moving the torch rightward over a distance of about 20 mm while moving the torch downward at the constant speed so that the arc length LC at xe2x80x9cCxe2x80x9d point is 0.3 mm.
In FIG. 8B, the horizontal axis represents welding modes or conditions xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d and xe2x80x9cCxe2x80x9d, and the vertical axis represents the measured voltage. Under the welding condition xe2x80x9cAxe2x80x9d where the arc length is 0.3 mm, the voltage is in a range of 10.2 to 10.8 V. Similarly, under the welding condition xe2x80x9cBxe2x80x9d where the arc length is 1.0 mm, the voltage is in a range of 10.0 to 10.6 V. Under the welding condition xe2x80x9cCxe2x80x9d where the arc length is 0.3 mm, the voltage is in a range of 10.2 to 10.8 V. Although there is a little variation, the measured voltage is nearly constant even through the arc length varies from 0.3 to 1.0 mm. It may be safely said that the proportional relationship between the arc length and the voltage is not established.
FIGS. 9A and 9B are graphs showing correlations between the arc length and the voltage that were obtained through an experiment using continuous arcs. In the experiment, welding of a copper plate was performed using a continuous arc produced by a TIG arc welding machine having a 1.6-mm-diameter tungsten electrode.
FIG. 9A shows the experimental result obtained when the continuous arc welding was achieved at a current of 105 A. In this figure, the horizontal axis represents the arc length and the vertical axis represents the voltage. It can be seen from FIG. 9A that the arc length and the voltage are proportional to each other and they substantially assume a linear function.
FIG. 9B shows the experimental result obtained from the continuous arc welding achieved at a current of 14 A. As seen from this figure, the arc length and the voltage are proportional to each other as long as the arc length indicated by the horizontal axis is not less than 1.0 mm. When the arc length is below 0.5 mm, the voltage shows an increase. As a consequence, there is no proportional relationship established between the arc length and the voltage when the arc length is less than 1.0 mm. This confirmed that the arc voltage control is not suitable for a continuous arc welding process using a small current.
The present inventors have reached a view that the known proportional relationship between the arc length and the voltage can be used when the welding is achieved with a relatively long arc length (larger than 1.0 mm and in a range of 2.0 mm to 5.0 mm) and at a relatively high current value (larger than 100 A), but the same relationship cannot apply when the arc length is less than 1.0 mm.
On the other hand, from the energy saving and environmental preservation point of view, very thin sheet metals have found an increased use in the manufacture of vehicle bodies and exhaust systems. For the manufacture of such vehicle bodies and exhaust systems, a welding process that can be achieved at a relatively small current not much exceeding 20 A and with an arc length of about 0.5 mm have become absolutely necessary.
The present invention provides a novel welding technique which can realize welding of very thin sheet metals that cannot be achieved by the conventional arc voltage control.
In groping a weld control technique replacing the conventional arc length control and arc voltage control, the present inventors found a fact that the arc light changes nicely as the distance between the electrode and the base metal varies.
According to a first invention, there is provided a control method for an arc welding process, comprising the steps of: (a) obtaining optical information from light produced from an electric arc during an arc welding process; (b) comparing the obtained optical information with reference information; (c) adjusting welding conditions on the basis of a result of comparison made between the obtained optical information and the reference information; (d) carrying out the arc welding process under the adjusted welding conditions; and (e) repeating the preceding processes (a) to (d) in the order named to thereby perform the arc welding process.
The control method of the first invention replaces the conventional constant voltage control or constant torch height control by a position control of an electrode or a torch that is performed while adjusting welding conditions on the basis of optical information obtained from the light of an electric arc produced during an arc welding process. The optical information, such as illuminance (also known as illumination or luminous flux density), obtained from the arc light has a good mutual relationship with the arch length or the torch height. Based on the illuminance or the like optical information, the weld control is achieved with high accuracy and reliability.
The arc welding process is a non-consumable electrode arc welding process. When used in such a non-consumable electrode arc welding process which is typified by the TIG (tungsten inert-gas) arc welding, the present invention can effectively perform welding of very thin sheet metals at low current and low voltage because a good mutual relationship established between the illuminance and the arc length has been confirmed even when the arc length is 0.25 mm. This enables a low heat exertion welding process, which can sufficiently suppress distortion of the base metal.
The optical information comprises illuminance of the arc light. The optical information may include illuminance of the arc light (illuminance at all spectrums), a variation of illuminance, spectral distribution, spectral intensity and so on. Among others, the illuminance can be easily measured by an illuminance meter, which is readily available, so that by reducing the invention method into practice, a substantial cost increase can be suppressed.
According to a second invention, there is provided an arc welding apparatus comprising: a welding machine that produces an electric arc across an electrode and a base metal; an optical sensor that obtains optical information from light produced from the electric arc during an arc welding process; and a processing unit that adjusts welding conditions according to the obtained optical information.
As described above, the optical information, such as illuminance, obtained from the arc light has a good correlation with the arc length or the torch height. Thus, the weld control is carried out on the basis of the optical information such as illuminance. The apparatus used for achieving the weld control merely requires an optical sensor and a processing unit that are provided in addition to an existing welding machine. Accordingly, undue increase in the equipment cost can be suppressed.
The arc welding process achieved by the arc welding machine is a non-consumable electrode arc welding process. When the present invention is embodied in such a non-consumable electrode arc welding process typified by the TIG welding, it is possible to realize welding of very thin sheet metals at low current and low voltage because a good mutual relationship established between the illuminance and the arc length has been confirmed even when the arc length is 0.25 mm. This ensures that high quality welded parts can be obtained.
It is preferable that the arc welding apparatus uses optical information comprised of illuminance of the arc light because an inexpensive and readily available illuminance meter can be used. Incorporating such illuminance meter into the arc welding apparatus does not greatly increase the equipment cost.